The present invention relates to a method and apparatus for optically detecting transient motion from a scattering surface. The invention is particularly directed toward detecting optical phase modulations such as those produced by low frequency ultrasound which is very useful for probing coarse microstructure materials.
The detection of the phase modulation or frequency modulation of an optical wave is important for various fields of application where optical beams are used to detect the motion of objects. This is the case of laser sensing of vibrations and laser detection of ultrasound and of transient body deformations such as those produced by a shock or on impact. Of particular interest for practical applications is the case where ultrasound or a shock wave is generated by a laser. In this case, a completely remote ultrasonic inspection system can be realised, permitting for example ultrasonic probing at elevated temperatures. A technique based on laser generation and optical detection can thus be advantageously used to inspect materials at high temperatures (such as all metals and ceramics) for process and quality control, to detect flaws as soon as they are created during processing, to measure production parameters (thickness, temperature, etc.) and to determine microstructural properties on-line (grain size, porosity, etc.).
Ultrasound is generally produced by a high power laser which heats locally the surface of a sample or workpiece to produce an acoustic source, and the phase or frequency modulation can be detected by means of a laser interferometer. Since in many cases, the modulation excursions to be detected are small, sensitivity is a prime concern. Adequate sensitivity requires a receiving demodulating means which has a large effective light gathering efficiency. The poor sensitivity of most of the optical detection systems known to date is one of the main reasons that has limited the practical evolution of such a technology to full commercial application.
Generally, the light gathering efficiency of an interferometric system is characterized by its etendue parameter (or throughput), defined as the product of its effective entrance aperture area by the solid angle limited by the rays of maximum inclination passing through the entrance aperture center and thus defining the field of view. The maximum inclination rays can be defined as those which produce a shift of the interference pattern by a quarter of a fringe. The importance of this etendue parameter stems from its invariance within the frame of geometrical optics. A large etendue permits to choose light collecting optics of large size, being only limited by cost and practical feasibility, and to detect surface motion over a large area.
Also of prime concern for many applications is the capability of providing a frequency response representative of the exact surface motion. This can only be achieved if the detecting technique has a broad frequency bandwidth.
The effect of transient motion upon a laser beam scattered by a surface can be described in three different and equivalent ways. It can be said that the surface motion produces a variable phase shift or a Doppler shift of the instantaneous frequency, or generates sidebands on both sides of the laser frequency. In the case of pulsed ultrasonic excitation, these sidebands are broadened.
In order to provide sensitive optical detection of ultrasound or transient motion over a broad bandwidth, it has already been proposed in U.S. Pat. No. 4,966,459 to derive from the laser beam scattered by the surface a reference beam having a wavefront substantially matching the wavefront of the scattered laser beam and to cause this reference beam, after stripping it from its sidebands, to interfere with the scattered laser beam whose frequency spectrum includes the carrier laser frequency and adjacent sidebands. The reference beam is stripped from its sidebands with an optical cavity of the confocal Fabry-Perot type. Such an optical cavity, however, does not work very well at low ultrasonic frequencies since, at frequencies below 1 MHz, it cannot produce in practice a reference beam whose optical sidebands have been completely suppressed, while maintaining a large etendue or throughput.